Production of particulate solid-bearing low density air-permeable sheet materials

ABSTRACT

A method of producing a particulate-solid-bearing air-permeable sheet of material of other than woven and knitted material selected from non-woven fabrics and open cell foam materials includes the following steps: entraining a particulate solid in a gaseous carrier in the substantial absence of fibrous material; disposing one face of a preformed air-permeable sheet material, which material has a density at or below 0.25 g/cm 3 , in the path of a stream of the gaseous carrier and entrained particulate solid, whilst maintaining a pressure drop across the thickness of the preformed air-permeable sheet material from the one face to the other face of the air-permeable sheet material, whereby to entrap some or all of the entrained particulate solid on or on and in the air-permeable material; and fixing the retained particulate solid on or on and in the air-permeable material with a binder. The mean pore size of the preformed sheet material is greater than the mean particle size of the particulate solid.

FIELD OF THE INVENTION

The present invention relates to a method of producingparticulate-solid-bearing air-permeable sheet material capable ofachieving high concentrations of particulate solid and/or highair-permeability from a low density (0.25 gm/cm³ or less) preformednon-woven fabric or open cell foam.

BACKGROUND OF THE INVENTION

The incorporation of particulate solids in air-permeable sheet materialshas been practised for a very long time. Particulate solids may betherapeutic materials and may have an antiseptic or similar effect.Alternatively they may be adsorbents of gases (such as for exampleactive carbon particles, which have been incorporated both in fibroussheets and in cellular foam sheets).

Several methods have been adopted for incorporating particulate solidsin permeable sheet material. One of the simplest is to form a laminateof the particulate solid with two sheets of woven cloth by applying toone of them a free flowing powder before the lamination of the twosheets is effected. This method is however rather primitive and thepowdered material is not firmly bound but can shake out of the laminate.

Another method has been to impregnate a fibrous web, e.g. thin webs of anon-woven material with a suspension of the particulate material in asolvent carrier which also incorporates a binder (usually an aqueouslatex). This method however necessitates the use of very finely groundpowders since to maintain the powder in uniform suspension in thecarrier liquid (even with the aid of dispersing agents) is difficult ifthe particle size is too great. This method also suffers from thedisadvantage that the particulate solid loses some of its activity byprolonged contact with organic liquids in the suspension and may alsobecome coated with the binder itself thus preventing those particleswhich are so coated from being active.

Furthermore when the aqueous suspension is dried (usually by theapplication of heat) the normal migration of the binder from the centretowards the faces of the web tends to take fine particles with it sothat it is difficult with the microparticles used in this process toavoid such migration of the particulate solid. This leads toconcentration of the particulate solid adjacent to the faces of theresulting web where the binder is also concentrated by the migrationeffect.

A considerable amount of activity can therefore be lost and although aconsiderable weight of powder can be incorporated there is a limit tothe activity which can be achieved. Furthermore this process is veryexpensive to operate and is therefore rarely cost effective in acompetitive market.

It has also been proposed in the past to tackify the surface of thefibres in a non-woven fabric by for example applying a solvent whichplasticises the fibre surface or by heating to soften the surface ofthermoplastic fibres in the non-woven material. After tackifying it hasbeen proposed to apply a particulate solid such as an electricallyconductive material to the tackified surfaces as from a suspension in asuitable liquid and then to solidify the surfaces as by removal ofsolvent or cooling so that the particles of solid remain partiallyembedded in the surfaces of the fibre. In this way a substantial amountof solid particulate material can be incorporated in the non-wovenmaterial. Such tackifying methods are however also disadvantageous inthat deactivation of active particles can result and in that costs arehigh.

GB-A-2013102 describes a method of forming a filter material for use insafety clothing whereby adsorber grains initially on the surface of abase material are forced into the base material. An air current can beused to force the grains into the base material after the latter hasbeen wetted. Alternatively the grains can be placed onto the surface ofthe dry material and forced into position by violent vibration. Filtermaterial made by this method is stated to prevent the passage of liquiddroplets as well as of vapours.

EP-A-0272798 describes a method of reducing the penetrability of aporous material by selectively incorporating particles of a poremodifying agent within the larger pores. A suspension, dispersion oraerosol containing the pore-modifying agent is passed through thematerial-by establishing a pressure differential across the-thickness ofthe material. The difference in flow rate through the large and smallpores ensures that substantially all of the pore modifying agent istargetted to the large pores but it is critical that the pore-modifyingagent be applied to the material under conditions of low inertia toestablish such a flow pattern. The use of high inertia results in thepore modifying agent simply remaining on the surface of the materialbeing treated since it is not then able to follow the flow pattern intothe pores. The amount of pore modifying agent incorporated is describedas being generally insignificant (e.g.≦1% w/w) compared to the weight ofthe base material. It is preferred that the pore modifying agent be lessthan 5 uM in diameter.

GB-A-1421346, relates to the production of moulded fibreglass batting ofrelatively high density for sound and heat insulation uses bycompression moulding at elevated temperature of a fibrous battingcontaining a particulate thermosetting binder. In one step of themoulding process, particles of binder are entrained in a carrier fluidwhich is drawn through the fibrous batting by suction, particles whichbounce off the surface of the fibrous batting being collected in acollecting duct disposed above the batting. The density of the bindermaterial in the batting is increased by coating the fibres of thefibrous batting with water or oil before the carrier fluid withentrained binder particles is drawn through the batting.

The simultaneous formation of an air-permeable material from fibresadmixed with a particulate solid has also been carried out in the past.Thus paper-making methods (both those in the liquid phase and those inwhich air-laying is utilized) have been used in this way. Howeverdepositing a mixture of fibres and powder on a paper machine is adifficult process because of the viscous drag of water drainage on thepowders.

A more effective way of using fibres and particulate solid to prepare aparticulate solid-bearing-material is by an air laying technique inwhich a mixture of fibres and powder is deposited using an air stream.One such method is described and claimed in GB-A-1283721. The process ofthat patent allows heavyweight webs to be loaded with powder. Howeverthe upper loading limit is only about 70% by weight. (Percentage byweight as used throughout the present specification is with respect toloaded base material before any processing steps subsequent to theloading of base material with powder, such as the incorporation ofbinder, have occurred).

The method of GB-A-1283721 can be used for heavy weight i.e. 300 gramsper square meter at about 1 mm thick and upwards material (density from0.3 gm/cm²), the lower economical limit of web weight for this processbeing about 200 grams per square meter at 0.5 mm thick (density 0.4gm/cm³), at which weight not more than about 30% by weight of powder canbe incorporated. It is not suitable for use with synthetic fibres norfor lightweight webs having very high air or liquid flow rates throughthe resulting material. It is therefore not suitable for example forpreparing webs for use as air filters.

Using the method of GB-A-L 283 721 at below 200 gm per square meter(particularly with synthetic fibres) the webs are difficult to form, theamount of powder which can be incorporated falls rapidly (to well below30% by weight) and the rate of production falls to uneconomical levels.Furthermore, when this process is applied to the production oflightweight webs, the product uniformity (i.e. the evenness ofdistribution of powder in the lightweight web) is not satisfactory.

It has also been proposed to mix powders and a latex and then to foamthe latex as in GB-A-1471351. Webs made in this way are howevervirtually impervious to the flow of liquids and air and the activity ofthe powders is also impaired by the encapsulation of particles ofpowder.

Only a few of the prior art procedures are applicable to low densitymaterials and even those that are either lower the activity of theparticulate solid or are far too costly for general use.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofproducing particulate-solid-bearing air-permeable sheet materials fromlow density air-permeable sheet materials which may be below 1 mm inthickness, which enables high air and liquid flow rates to be achievedin the resulting particulate-solid-bearing air-permeable material andalso enables high (e.g. over 80% by weight) loading of particulate solidon or on and in the air permeable sheet material to be made economicallywithout substantial loss of activity of the particulate solid material.

This object is achieved according to the invention by providing a methodof producing a particulate-solid-bearing air-permeable sheet material,which comprises: entraining a particulate solid in a gaseous carrier inthe substantial absence of fibrous material; characterised in that themethod comprises disposing one face of a preformed air-permeable sheetmaterial selected from non-woven fibrous materials and open cell foammaterials, which material has a density of at or below 0.25 g/cm³, inthe path of a stream of said gaseous carrier and entrained particulatesolid, whilst maintaining a pressure drop across the thickness of thepreformed air-permeable sheet material from said one face to the otherface of said air-permeable sheet material, whereby to retain some or allof the entrained particulate solid on or on and in the air-permeablematerial; the mean pore size of the pores of the preformed sheetmaterial being greater than the mean particle size of the particulatesolid and fixing the retained particulate solid on or on and in theair-permeable material with a binder.

The pore size of a pore is the diameter at the smallest cross-section ofthat pore. The mean pore size of a sheet material is the average of thepore sizes of the pores present in that sheet material. The meanparticle size of a particulate material is the average diameter of theparticles of that material.

It has surprisingly been found (contrary to expectation) that retentionof the particulate solid is such that % w/w loadings of particulatesolid of over 80% (about 90% in some cases) can be achieved with webshaving a density at or below 0.25 g/cm³ whilst maintaining high airpermeability, and even at a thickness below 1 mm. It would have beenexpected that such webs would retain very little, if any, of theparticulate solid.

As will be illustrated hereinafter in more detail with reference to FIG.11 of the drawings the effect of using a particulate solid with aparticulate size greater than the mean pore size of a sheet material hasbeen found to be blocking of pores and substantial surface depositswhich gives uneven distribution of particles in and on the sheetmaterial. In contrast, the method of the invention permits distributionof particles in or in and on the sheet material to be concurrentlycontrolled both over the lateral dimensions of the sheet material andacross its thickness as desired coupled with high permeabilty for agiven loading.

The present invention need not always be used to achieve loadings atthis level. Loadings of from e.g. 10, 20 or 30% w/w to e.g. 50, 70 or90% w/w (in some cases) by weight can be produced as desired.

The preformed air-permeable sheet material can have a density as low as0.005, but is preferably equal to or greater than 0.007 and mostpreferably equal to or greater than 0.010. Whilst the material may havea density as high as 0.25g/cm³ preferably the density is equal to orless than 0.18 and most preferably equal to or less than 0.15.

The present invention can be used in applications where air permeabilityis important e.g. in the manufacture of filters.

The air permeability will depend on the density of the preformednon-woven fabric or foam, the loading of active particulate solid andthe amount of binder incorporated. These parameters are chosen accordingto the intended end use so that the permeability achievable will dependlargely on the end use of the product. For a given end use, the methodof the present invention can achieve significantly higher permeabilitiesthan those of products made by prior art methods.

As hereinafter explained the present invention can, for a given loadingor particulate material, be used to produce loaded base material with ahigher air permeability than prior art methods.

Thus in Example 1 the present invention produces a product with a powdercontent of 99 g/m² and an air permeability of 100 L/min/10 cm² ofproduct, using a 10 mm water gauge.

Using the same particulate material, the method of GB-A-1283721 producesa product with a powder content of 80 g/m² but with an air permeabilityof only 76 L/min/10 cm² of product using a 20 mm water gauge (Seecomparative Example 1b). Thus, the present invention, for approximatelythe same loading of particulate material in this instance gives abouttwo and a half times the air permeability achieved by the prior art.

The pressure drop may be effected by maintaining a lower gaseouspressure at said other face of the air-permeable sheet than that extantat the said one face of the air-permeable sheet. Preferably this iseffected by applying suction to said other face of the air-permeablematerial.

Thus one embodiment of the present invention is a method as hereinbeforedescribed wherein a stream of the gaseous carrier and the particulatesolid in substantially free flowing form are supplied to a mixing zone;the substantially free-flowing particulate solid material is entrainedin the mixing zone in the stream of the gaseous carrier; the mixture ofgaseous carrier and entrained particulate solid from said mixing zone ispassed into the inlet of, through and out of, the outlet of a supplyzone; a suction zone of variable effective length and width isestablished adjacent and in line with the outlet of said supply zone theeffective width and length of said suction zone being greater than theeffective width and length of said supply zone; the pressure in saidsuction zone is reduced to below that at the outlet of said supply zone;and the air-permeable sheet material is continuously fed between saidsupply zone and said suction zone. Substantially no fibrous material isentrained in the gaseous carrier.

Whilst the present invention-can utilize carrier gases such as nitrogenand carbon dioxide, the preferred carrier gas is air. Thus the method ofthe present invention a method as hereinbefore described comprisingestablishing a recirculatory flow of air; wherein a stream of air atsuper atmospheric pressure is passed through the mixing zone and theninjected into the supply zone, which supply zone is maintained atatmospheric pressure; air from said supply zone is sucked into thesuction zone, which suction zone is disposed adjacent and in line withsaid supply zone and is maintained at sub-atmospheric pressure; air fromsaid suction zone is raised to super-atmospheric pressure and is fed tosaid mixing zone; and the free-flowing particulate solid is fed to saidmixing zone.

Whilst the method of the invention can be effected in a batchwise mode(individual sheets of air-permeable material being treated successively)it is preferred for reasons of economy to operate the method on acontinuous basis, using a continuous length of air-permeable material.Thus a continuous length of air-permeable sheet material may be passedbetween the aforementioned supply and suction zones.

The mean pore size is greater than the mean particle size, preferably atleast twice the mean particle size.

Wide ranges of particle sizes and bulk densities of particulate solidare suitable for use in the present invention since it has surprisinglybeen found that subject to the above relationship between mean pore sizeand mean particle size these factors have only a small effect on theincorporation of particulate solid. Suitable particle sizes are e.g.from 0.5 um to 400 um, more specifically from 8 uM to 250 um.

Suitable bulk densities are e.g. from 0.14 to 2.52 g/cm³ morespecifically from 0.3 to 0.67.

Although the density of the air permeable sheet material is an importantfactor affecting the incorporation of particulate solid, it hassurprisingly been found that subject to the above relationship of meanpore size to mean particle size the pore size is relatively unimportantand wide ranges of pore sizes e.g. from 10 to 1000 uM, more specificallyfrom 50 um to 1000 uM, can be used.

The flow rate of the gaseous carrier through the air-permeable materialdoes affect the incorporation of particulate solid, the flow rate inturn depending on the pressure drop across the thickness of theair-permeable material i.e. on the suction applied. The amount ofparticulate solid incorporated also depends on the time during which theair-permeable material is exposed to the flow of gaseous carrier andentrained particulate solid, e.g. to the dwell time of the continuoussheet between the supply and suction zones. Depending on each of theseparameters the amount of the entrained particulate solid which isretained on or on and in the air-permeable sheet may be varied from allthe entrained particulate solid to only a proportion of the entrainedparticulate solid. Where only a portion of the entrained particulatesolid is entrapped on and/or in the air-permeable sheet, that portionwhich passes through e.g. into the suction zone may be recirculated to(albeit another portion of) the air-permeable material again.

When a recirculating system is employed, air which bleeds into thesupply zone from outside passes through the sheet material and can beremoved from the suction zone via a separate outlet to that provided forrecirculation.

Air-permeable materials especially those composed of non-woven fabricsand open cell foams do not in general have a perfectly uniformdistribution of voids throughout the material. Consequently thedistribution of the particulate solid throughout the air-permeable sheetcan never be absolutely uniform. In practice, absolute uniformity ofdistribution is not however necessary for most applications of theproduct. A measure of uniformity of distribution in the plane of thesheet is the variation in weight of particulate solid contained withinpanels of a given area cut out from the sheet at intervals. Thus acontinuous sheet material can be notionally divided up into stripsextending laterally across the whole width of the sheet and having apredetermined width measured along the continuous axis of the sheet.Thus a variation of ±10% in the weight of particulate solid from onestrip to another might be regarded as acceptable. The actual width ofthe strips analysed is however a crucial factor in determiningvariability of particle distribution. Thus a 10% variation as betweenstrips 10 cm wide might be achieved, but if the measurements are made onthe same sheet with 1 cm strips the variation could exceed 10% asbetween the 1 cm strips by a substantial margin. The method of thepresent invention can achieve a distribution of particulate material inthe plane of a sheet wherein the weights of particulate material inpanels having a smallest surface dimension of 10 cm and of equal area,vary by less than about 10%.

The distribution of particulate solid across the thickness of the sheetis not necessarily uniform. If it is desirable to have a lowerconcentration of particulate solid on or adjacent to one face of thesheet, this can be achieved by adding water to the air-permeable sheetmaterial to the extent that interconnecting voids in the air-permeablematerial are filled or partially filled with water before one facethereof is subjected to the stream of carrier gas and entrainedparticulate solid. The effect of this is to cause the particulate solidto be deposited predominantly on the one face and in the voids adjacentto that one face until the effect of the pressure drop clears the voidsof water. The effect is progressive and is also affected by the otherparameters such as the gradient of the pressure drop and the duration ofits application. This contrasts with the mere wetting of the fibresurfaces in a non-woven material with water or oil described in GB-A-1421 346.

Thus in the continuous process, by selection of suitable values for theapplicable parameters, the particulate material may be concentrated onthe one surface with little particulate solid within (i.e. away from theBaid surface of) the air-permeable sheet; or the concentration ofparticulate solid may vary continuously across the thickness of thesheet from a high value at the one face to a lower value at the otherface of the air-permeable material or may be substantially uniformacross the thickness of the material. In determining the gradient ofconcentration across the thickness of the sheet, desirably averagevalues should be determined over a reasonable large area of the sheet(e.g. 10 cm squares).

The method of the present invention has a number of advantages overprior art methods. It lends itself to continuous operation, results inlittle or no loss in the activity of incorporated active particles andcan provide products having high levels of permeability to gases andliquids as well as being capable of enabling high levels of particulatesolid (70% w/w based on dry weight of air permeable sheet beforeincorporation of binder material) to be achieved.

Preformed air-permeable materials for use in the present invention arenatural or synthetic fibrous non-woven materials and open cell foams.More specific air-permeable materials are given in the Exampleshereinafter described.

The air-permeable sheet may be of any desired thickness consistent withthe desired air-permeability. Thus, whilst thicknesses of from 0.1 mm to0.50 mm can be used, which contrasts with the practical lower limit ofabout 0.5 mm thickness in the process described in GB-A-1283721, thickermaterials up to about 50 mm can be processed by the method of thepresent invention.

Binders for use in the present invention may take the form of natural orsynthetic latexes, e.g. natural rubber latex, NEOPRENE, styrenebutadiene, acrylonitrile butadiene, acrylic methyl methacrylatepolyvinyl alcohol, polyvinyl acetate, melamineformaldehyde resins, orthey may comprise solutions of starch, carboxymethyl cellulose, methylcellulose or sodium silicate. The latexes may be aqueous.

The binder may be applied to the air-permeable material containingparticulate solid by means of an applicator roller which receives thebinder solution or suspension from a reservoir via a spreader whichspreads the binder over the surface of the applicator roller. The binderis preferably applied in excess; the excess liquid being caught by atray disposed underneath the air-permeable material which is beingimpregnated; the excess being recirculated to the binder reservoir. Theamount of binder liquids may be further reduced by passing theimpregnated air-permeable material over a suction box or through niprolls. Preferably the air-permeable material is held between two wiremeshes during the impregnation with the binder liquids. Theair-permeable material impregnated with binder then passes to a dryingsection where the material is dried.

Alternatively the binder can be applied by spraying, padding, laying offoam and using suction or any other conventional method. If a watersoluble binder e.g. polyvinyl alcohol is used, the polyvinyl alcohol inpowder form can be entrained in the gaseous carrier together with theparticulate solid and deposited in the preformed air-permeable materialby the method of the invention. Impregnation of the material withsufficient water to dissolve the polyvinyl alcohol particles can then beused to form the binder in situ.

Drying of the impregnated material can then be effected, using anyconventional means for example hot air, radiant heat, heated cylindersetc.

It is also possible to use a thermoplastic binder, in which case theparticles of thermoplastic binder can be entrained with the particulatesolid to be deposited in and/or on the air-permeable material andresulting material can be passed over heated cylinders or the like toeffect melting the thermoplastic particles and bonding of theair-permeable material.

The preferred method of operating the invention however is with latexbinders.

One of the advantages of the method of the present invention is thatcontact between the particulate solid and the liquid binder need becomparatively brief e.g. for less than 60 seconds. The risk ofencapsulation of the particles of solid material with binder istherefore low and in the practice of the present invention very littlesuch encapsulation takes place. Furthermore, the size of particles whichcan be used in the method of the present invention can be large enoughto prevent migration of such particles with migration of the binder. Afurther advantage is that non-compatible binders can be used in themethod of the invention (i.e. binders which would precipitate out ofsolution or suspension on prolonged contact (e.g. contact of over 60seconds) with the particulate solid. Incompatible binders cannot be usedin those prior art methods where such prolonged contact would obtain.Such problems are not evident in the method of the present invention.

The method of the present invention allows the use of a wide range ofbinders to meet different circumstances without materially increasingthe cost of the production.

When entraining powdered materials in gaseous carriers, particularlywhen the gaseous carrier contains oxygen, it is desirable to include asufficient amount of moisture so as to prevent the build up of static orto prevent flashing in the event that a flammable powder is entrained.The powder or powder/air (e.g. carbon or carbon/air) mixture can contain20 to 55% w/w/ e.g. 25 to 35% w/w of moisture (with respect to the drypowder). The amount of moisture should not prevent the particulate solidfrom being substantially free flowing. The moisture can be added to thepowder (if necessary) where it is possible to absorb the moisture withinthe powder particles themselves, or it can be added to the gas streamfor example by injection of steam. Depending upon the humidity of theexternal environment, the moisture required to be added will vary. Inenviroranents of high humidity for example, it will not be necessary toadd so much moisture as in environments of low humidity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated, without limitation thereof, byway of Example, with reference to the following diagrams, wherein:

FIGS. 1 and 2 illustrate schematically a process whereby the method ofthe present invention can be best performed, FIG. 1 showing a part ofthe process in which particulate material is loaded onto a base materialand FIG. 2 a part of the process in which binder is used to fix theparticulate material;

FIG. 3 is a detailed view of one form of apparatus suitable forperforming the present invention;

FIG. 4 is an end view of the apparatus shown in FIG. 3,

FIG. 5 is a plan view of a part of the apparatus shown in FIG. 1, and

FIG. 6 is a section along the line A--A of FIG. 5.

FIGS. 7,8,9,10 illustrate the relationship between density of basematerial (g/cm³) along the x axes and % loading of particulate materialon and in the base material (along the y axes), using differentparticulate materials and using the apparatus illustrated in FIGS. 3 to6.

FIG. 11 shows a cross section of a pore of a base material which hasbeen loaded with particles of a greater mean size than the mean size ofthe pores of material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, particulate material is fed at a predetermined ratethrough inlet 2. The particulate material passes through inlet 2 intosettling chamber 6. The particulate material is deposited on or in andon a preformed non-woven base material (hereafter "web") which issupplied from unwind 1 and which is supported upon a moving continuouswire-mesh 8. Wire mesh 8 bearing the web passes across the open base ofthe settling chamber 6, preferably approximately 1.2 m below the entryof the particulate material from inlet 2. It then passes over rollers 10and 12, roller 10 being driven by a motor (not shown) thus driving thewire-mesh 8. If the preferred web has sufficient strength the wire-meshcan be dispensed with and the preformed web passed directly onto suctionbox 14.

Suction box 14 is disposed beneath the open base of the settling chamber6 so that the wire-mesh 8 passes between the settling chamber 6 and thesuction box 14. The top of the suction box 14 may comprise a series of2.5 cm wide grooved wooden frame members 16 spaced 5.1 cm apart thusleaving a series of 5.1 cm wide openings 18. Each of the openings 18 isprovided with one or two slats 20 slidably mounted between each pair ofadjacent frame members. Two slats are provided where it is required tovary the effective area of suction, one slat being provided when thesuction is not required and the slat is normally closed. Suction isprovided by two fans of adjustable speed (not shown) which draw air outof the suction box 14 via outlets 22 and 24. The air extracted from thesuction box 14 via outlet 22 is recirculated and is used to carry thefeed to inlet 2. The air withdrawn via outlet 24 is the excess air whichhas leaked in or has deliberately been allowed in as described below.The amount of air blown into the settling chamber 6 may be controlled byadjustment of the flow of recirculated air.

Two walls 26 and 28 of the settling chamber 6 may be adjustable thusenabling a certain amount of air to be led onto the web or allowing airto be passed directly into the left-hand end of the suction box 14.

A gap 28a may also be provided at the bottom of wall 28 to bleed airinto the chamber. This gap (when present) may be sealed by roller 28b.

The loaded web 36 then passes to the binder impregnation stage (see FIG.2).

The loaded web 36 (if not strong enough to be self-supporting) issupported on a moving continuous wire-mesh 38 which passes over rollers40,42 and 44. The web 36 still supported by the wire-mesh 38 passesunder roller 46 where it is impregnated by liquid drawn from reservoir48 and spread on the roller 46 by means of a spreader 50. Excess liquidflows into tray 52 and is recirculated by pump 54 via pipe 56 to theliquid reservoir 48. A second moving continuous wire-mesh 58 may beprovided which passes round rollers 46,60,62 and 64. Thebinder-impregnated web 36 is then held firmly between the wire-mesh 58and the first wire-mesh 38 as it leaves roller 46. The two wire-meshes38 and 58 may be tensioned or slackened, depending upon whether it isdesired to compress the web or not. Tensioning is effected by adjustmentof rollers 42, 46 and 60.

Further excess liquid is removed from the web by a suction box 66,passed to a separator and then recirculated via tray 52 , pump 54 andpipe 56 to liquid reservoir 48.

The wire-meshes in the impregnator, where present, are kept clean byspraying them with water at appropriate intervals. The impregnated websupported by continuous wire-mesh 72, which passes over rollers 74 and76, passes through infra-red tunnel 78 which is filled with infra-redlights 80. The web is thus dried by the infra-red lights 80 and bycirculation of air through the tunnel.

If desired the web may then be passed over a series of drying cylinders82.

In FIGS., 3, 4, 5 and 6 the corresponding numbers used in relation toFIGS. 1 and 2 are used to designate similar parts.

Referring-to FIGS. 3 and 4, part of the apparatus is mounted on a framecomprising a number of angle irons 90,92,94,96,98,100,102. A feed hopper104 is connected via its outlet and a rotary valve 229 to pipe 22, aVIBRA-SCREW feeder 106 being provided in the outlet of the hopper. Pipe110 connects pipe 22 to inlet 2, a fan being disposed at point 108 inpipe 110.

Side walls 113 an 115 may be slid along bar 216 to vary the width ofsettling chamber 6 (for different widths of base material).

A continuous wire-mesh 8 (if required for support of the base material)is mounted on drive roll 10 and tensioning rolls 12 and 122, the tensionbeing adjusted by tightening or loosening screws 118 attached to frame120 carrying roller 122. The drive roll 10 is driven by a variable speedmotor 124 through a gear box 126.

A suction box 14 is disposed below the settling chamber 6. Two outletpipes 22 and 24 are provided in the suction box, the outlet pipe 24leading to fan 128 and through to a bag filter (not shown) via abutterfly valve (not shown).

In operation, particulate material is fed from the hopper 104 by thescrewfeeder 106, (for example, a VIBRA-SCREW feeder to the pipe 22 byway of rotary valve 229, where it is mixed with the air flowing in thepipe 22 in a mixing zone downstream of the valve 229. The solidparticulate material is entrained in the stream of the air in the mixingzone in the pipe 22 downstream of the valve 229. It has been foundexpedient to incorporate some form of a rotary seal between hopper 104and pipe 22 to prevent variations in the feed from occurring and also toprevent leakage into the system of more air causing pressure variations.

The particulate solid material entrained in the air flowing in the pipe22 is then passed via the fan 108 and the pipe 110 into the inlet 2 tosettling chamber 6. Such material and air pass downwardly through thesettling chamber 6 via an uninterrupted path to the web supported uponcontinuous wire-mesh 8. The suction box 14 provides a pressuredifferential across the web and thus serves to maintain the web incontact with the wire-mesh 8. Air is drawn from the suction box throughpipes 22 and 24. The air removed through pipe 22 is recirculated withmore particulate material fed into pipe 22 from the hopper 104.

The amount of air removed through pipe 24 may be regulated depending onthe amount of air bled into the settling chamber. The hinged walls 26and 28 of the settling chamber may, if desired, be raised at their lowerends to allow air to bleed into the settling chamber at these points.

The web is carried out of the settling chamber 6 after deposition withparticulate material and is removed from the mesh 8 as it passes overdriving roll 10 and may then be further treated as desired.

Referring to FIGS. 5 and 6, suction box 14 may comprise a number ofgrooved wooden frame members 16 (usually 2.5 cm in width). A pair ofslats 20 comprising tongues 136 are slidably mounted between eachadjacent pair of the wooden frame members 16. Where the openings 18 arerequired to be closed a single slat may replace the pair of slats.

An opening 138 is provided in the base of the tray 134 through which airis removed from the suction box. The opening communicates with outletpipe 22 at the end of which is disposed a fan (not shown). A secondopening in the suction box 14 which connects with pipe 24 is not shownin FIGS. 4 and 5.

Two slats 20 are slidably disposed in each of the gaps 18 between framemembers 16 of suction box 14 so that these openings 18 may be opened orclosed as desired by lateral movement of each pair of slats outward orinward as the case may be. In general since only the first half dozen orso of the slats 20 used need to be in the open position, the remainingpairs of slats are preferably replaced by a single slat covering thewhole width of the suction chamber. Usually one or other of the last twogaps 18 in the sequence adjacent the forward end of the suction box areleft open.

In the following examples part of the air from the suction box wasimmediately filtered by a filter bag for re-use. Unless otherwisestated, no moisture was added to the particulate material since most ofthe examples were performed in short runs and therefore the risk offlashing was low. Various particulate materials were loaded onto variousbase materials and were fixed using a binder. Mean pore size wasdetermined using The Bubble Pressure Test described in British standardNumber BS 3321:1960.

Mean particle size was determined using the Malvern Method. This methodis described in USA Standard MBS PART 52577. For each of thesesubstances, the supplier is given below, together with the trade nameunder which that particular product can be obtained from the supplier.For the base materials the particular example number or comparativeexample number in which the material was used is given. ((C) indicates acomparative example).

    ______________________________________                                        Particulate Material                                                          Trade Name      Supplier                                                      ______________________________________                                        Grade C Carbon  Chemviron Carbons Limited,                                                    113 High Street, Uppermill,                                                   Oldham, Lancs. OL3, 6BD                                                       England                                                       Grade CA-1 Carbon                                                                             Norit (UK) Limited                                                            Clydesmill Place,                                                             Cambusland Industria1 Est.                                                    Glasgow, G32 8RF, Scotland.                                   Silica Gel and Gasil GM2                                                                      Crosfield Chemicals Limited                                   A Quality       Bank Quay Works, Warrington,                                  -0.25 mm        Cheshire WA5 1AB, England.                                    Super Absorbent Material                                                                      Stockhausen GmbH                                              Favor SAB 922   T Division, Bayer House.                                                      Manchester Road, Altrincham                                                   Cheshire WA14 5PF, England.                                   ______________________________________                                        Binder Trade Name                                                                             Supplier                                                      ______________________________________                                        Acronal LA471S  B.A.S.F. Plc, Earl Road,                                                      Cheadle Hulme, Cheadle,                                                       Cheshire, SK8 6QG, England.                                   ______________________________________                                        Example or                                                                    Comparative                                                                            Base Material                                                        Example No.                                                                            Trade Name    Supplier                                               ______________________________________                                        5(C)     Chemically Bonded                                                                           Porvair Limited                                                 Nonwoven Porvair                                                                            Estuary Road, King's Lynn                                                     Norfolk, PE30 2HS, England                             1*,12*,20*,                                                                            Chemically Bonded                                                                           Bonar Carelle Limited,                                 33*,11,16,                                                                             Polyster Bonella                                                                            Nobel Road, Geordie,                                   31,37,   Ultraloft 95 g/m.sup.2                                                                      Dundee, DD2 4UH, Scotland.                             3,34,13,23                                                                             Needled Polyester                                                                           Tharreau Industrie,                                             Dutexim 41-06 z.l. de la Pierre Blance                                        60/m.sup.2    P.B. 49-49120, Chemille,                                                      France                                                 17,32,38 Thermally Bonded                                                                            Camtex Fabrics Limited,                                         Fabric from   Blackwood Road,                                                 Heterofil Fibre N                                                                           Lillyhall North,                                                17 Silver Grey                                                                              Workington,                                                     Pique         Cumbria CA4 4JJ,                                                              England.                                               la(c)    Cotton Scrim  Whiteside Mfg. Co. Limited                                      44 × 36 bleached                                                                      Thames Industrial Estate.                                                     Higher Ardwick, Manchester                                                    M12 6BZ, England.                                      10,15,30,36,                                                                           Chemically Bonded                                                                           Mansell Bonded Fabrics,                                         Viscose 16 g/m.sup.2                                                                        Unit 2, Hythe Quay,                                             Nonwoven      Colchester CO2 8JB,                                                           England.                                                        Polyester High Bulk                                                                         Jute Webberei Emsdetten                                         Fleece        GmbH,                                                  18       JW60          Postfach 1455,                                         19       JW100         Rheiner Str. 125, D4407                                         Warren Gruppe 50                                                                            Emsdetten,                                                      2731 Qualitex Germany.                                                        High Bulk Fleece                                                              60 g/m.sup.2 and                                                              100 g/m.sup.2                                                        28       Needled and Chemi-                                                                          Libeltex NV                                                     cally Bonded, Marialoopsteenweg 51,                                           Polyester (L3)                                                                              B-8860, Belgium.                                                Liplisse 3 100 g/m.sup.2                                             9,29     LD32 Chemically                                                                             Ledacare Limited                                                Bonded Polyester                                                                            Longshaw Industrial Park                                        No. 201       Highfield Road,                                        2,22     LD58 Bulked   Blackburn,                                                      Chemically Bonded                                                                           Lancs BB2 3AS                                                   Polyester No. England.                                                        R13416B                                                              5,24     LD65 Mechanically                                                             Entangled Polyester                                                           Malifleece 3/4 D                                                              Tex 65 g/m.sup.2                                                     8,27     LD73 Bulked                                                                   Chemically Bonded                                                             Polyester Ref.                                                                R13444C                                                              7,26,    LD90 Mechanically                                                             Entangled Polyester                                                           Malifleece 12 D Tex                                                           96 g/m.sup.2                                                         6,25,35, Polyurethane Foam                                                                           Caligen Foam Limited                                                          Broad Oak, Accrington,                                                        Lancs. BB5 2BS, England.                               4,14,    Needled Bonded                                                                              Lohmann UK Limited                                              Polyester Paramoll                                                                          Credsec House,                                                  VN413A        Oxford Road, Stone,                                                           Aylesbury,                                                                    Bucks. HP17 8PL,                                                              England.                                               21       Blue Lofted   Midland Filter Products                                         Nonwoven      Limited, Building 6,                                                          Cavalry Hill, Weedon,                                                         Northants, NN7 WPS                                                            England.                                               1b(c)    Polypropylene Net                                                                           Smith and Nephew Plastics                                       GS 3736       Limited, Gilberdyke,                                                          North Humberside,                                                             HU15 2TD, England.                                     3(c)     Fibre Glass Mat                                                                             W. David & Sons Limited                                         (Davids) Fastglass                                                                          Donington Ind. Estate,                                          from Japan)   Wellingborough,                                                               Northants, England.                                    2(c)     Paper (R36-00501                                                                            P. Garnett & Son Limited,                                       Yellow Brown  Wharfeside, Otley,                                              Gumming Kraft)                                                                              W. Yorkshire, LS21 1QJ,                                                       England.                                               ______________________________________                                         *These materials when obtained from the supplier but before use as base       materials, where expanded by briefly heating until they were of the           calipers shown in the Examples.                                          

Binder was used as an aqueous solution at the % w/v figures given belowfor the given Examples:

    ______________________________________                                        Example No.         % w/v of Binder                                           ______________________________________                                         1 to 11            7%                                                        12 to 17            3.5%                                                      18 to 38            9%                                                        Comparative Example 1a                                                                            9%                                                        Comparative Example 1b                                                                            8%                                                        Comparative Examples 2 to 5                                                                       9%                                                        ______________________________________                                    

EXAMPLE 1

This example was performed using the apparatus illustrated in FIGS. 3,4, 5 and 6 above and using silica gel as the particulate material. Thesilica gel had a mean particle size of 110 um and a bulk density of0.674 g/cm³. It was loaded into the hopper 104 and was passed throughthe inlet 2 into the settling chamber 6 at a rate of 162 g/min, whilst abase material, of 330 mm width which was supported upon the wire meshwas passed over the suction box at a feed rate of 1 meter per minute.The base material had a weight of 95 g/m², a caliper of 9.8 mm and anair flow of over 100 liters/min/10 cm² at 1 mm water guage. The meanpore size of the base material was 764 um. The silica gel particles wereloaded onto the base material. The loaded base material was thentransported to the impregnator where it was impregnated with ACRONALLA471S binder. Excess binder was removed partly by squeezing theimpregnated material between the two wire meshes of the impregnator andpartly by suction from a suction box situated below the lower wire meshof the impregnator. The impregnated material was then partially dried inan infra-red tunnel and then passed over a bank of paper-dryingcylinders for further drying. The resultant product had a weight of 135g/m² and a thickness of 4.4 mm. Its density was 0.031g/cm³ and it had anair permeability of 65L/min when measured on a 10 cm² piece of materialusing a pressure of 1 mm water gauge. Its mean pore size was 732 um andits Latex content and particulate material (powder) content were 4.0g/m² and 34 g/m² respectively. The percentage powder content wasdetermined for this as for the other Examples and Comparative Exampleswith respect to weight of the product less its binder content (i.e. withrespect to the weight of base material plus powder). The percentagepowder content was found to be 26% w/w. These results are set out inTable 1.

EXAMPLES 2 to 11

The procedure described in Example 1 was repeated but using differentbase materials. The properties of the base materials used and of theproducts obtained are given in Table 1.

EXAMPLES 12 to 17

The procedure described in Example 1 was repeated but using GASIL as theparticulate material (mean particle size 8 um; bulk density 0.437 g/cm³)and using the base material given in Table 2 (together with the resultsobtained).

EXAMPLES 18 to 32

The procedure described in Example 1 was repeated but using Grade Ccarbon as the particulate material (mean particle size 41 um; bulkdensity 0.435 g/cm³ and using the base materials given in Table 3(together with the results obtained).

EXAMPLES 33 to 38

The procedure described in Example 1 was repeated but using Grade CAlcarbon as the particulate material (mean particle size 35 to 40 um; bulkdensity 0.30 g/cm³) and using the base materials given in Table 4(together with the results obtained).

EXAMPLES 39 to 41

The procedure described in Example 1 was repeated but using the basematerials set out below and varying the procedure as set out below:

EXAMPLE 39

A needled polyester nonwoven web with a basis weight of 65 g/m² and abulk density of 0.0215 g/cm³ was fed through the production machine at awidth of 1150 mm and a speed of 200 meters/hour. An active carbon powder(Grade C) was fed into the machine at the rate of 30 kg/hour. Theresultant carbon/nonwoven web was impregnated with a latex binderACRONAL LA471S of 5% w/w solids and then dried to give a pick up oflatex of 11% w/w.

The finished product which has the following properties was used for thepurification of air and the production of face masks.

    ______________________________________                                        Basis Weight  200 g/m.sup.2                                                   Caliper       1.20 mm                                                         Air flow      150L/min/10 cm.sup.2 /10 cm water gauge                         Pore size - mean                                                                            160 um                                                          Active carbon 114 g/m.sup.2                                                   content                                                                       ______________________________________                                    

EXAMPLE 40

The same conditions and materials as for Example 41 were used but thespeed of the machine was reduced to 160 meters/hour.

The resulting web with the following properties was used in multilayerwound cartridges for the filtration of water.

    ______________________________________                                        Basis Weight  225 gm/m.sup.2                                                  Caliper       1.25 mm                                                         Air Flow      90L/min/10 cm.sup.2 /10 cm water gauge                          Pore size - mean                                                                            134 um                                                          Active Carbon 142 g/m.sup.2                                                   Content                                                                       ______________________________________                                    

EXAMPLE 41

A needled and chemically bonded nonwoven web at a width of 1600 mm wasfed into the machine at a speed of 170 meters/hour. An active carbonpowder (Grade C) was fed into the machine at the rate of 42 kg/hour. Theresulting active carbon/nonwoven web was impregnated with a latex binderACRONAL LA471S of 12% w/w solids and then dried to give a latex pick upof 20%.

The finished web with a very high air flow and the following propertieswas used in air purification in air conditioning units.

    ______________________________________                                        Basis Weight  220 gm/m.sup.2                                                  Caliper       2.55 mm                                                         Air flow      500L/min/10 cm.sup.2 /10 cm water gauge                         Pore size - mean                                                                            352 um                                                          Active Carbon 88 g/m.sup.2                                                    Content                                                                       ______________________________________                                    

COMPARATIVE EXAMPLE 1a

The scrim described in Example 1 of GB-A-1283721 used as base materialin the process described in Example 1 above, but using the base materialat a width of 300 mm; using grade C carbon as particulate material,adding water to the carbon to bring its total moisture content to 40%(with respect to the dry carbon); and using a feed rate (the rate offeed of the carbon into the settling chamber) of 270 g/min. Most of thecarbon was concentrated on one surface of the product.

COMPARATIVE EXAMPLE 1b

The procedure described in GB-A-1283721 was carried out using apolypropylene net as support (as set out in Table 5) which was runthrough the machine at a width of 330 mm and a speed of 1 meter/min.,using a mixture of 20 parts by weight of polyester fibre and 90 parts byweight of grade C carbon as the particulate material; and using a totalfeed rate of the carbon and fibre to the settling chamber of 162 g/min.

Most of the carbon and polyester was concentrated on one surface of theproduct.

COMPARATIVE EXAMPLE 2

The procedure described in Example 1 above was repeated but using paperas the base material (as set out in Table 5) which was run through themachine at a width of 330 mm and a speed of 1 meter/min, using grade Ccarbon as the particulate material and using a feed rate of the carbonof 162 g/min. The paper used had a Bendtson air porosity of 450 (i.e.when a piece of the paper of 10 cm² area was tested using a 150 mm watergauge, a reading of 450 ml/min was obtained).

COMPARATIVE EXAMPLE 3

The procedure described in Comparative Example 2 was repeated but using250 mm wide glass fibre mat as the base material (as set out in Table 5)and using a feed rate of carbon of 123 g/min.

COMPARATIVE EXAMPLE 4

The procedure described in Comparative Example 2 was repeated but usingpolyurethane foam as an initial base material and using grade CAl carbonas the particulate material.

A product having a mean pore size of 256 um and a bulk density of 0.697g/cm³ and an active carbon content of 625 g/m² was obtained. Thisproduct (hereafter known as "carbon impregnated foam") was used as thebase material for a subsequent deposition of particulate material.

In the subsequent deposition, the procedure described in ComparativeExample 2 was repeated but using the carbon impregnated foam as the basematerial (as set out in Table 5) and using absorbent grains (SuperAbsorbent Material Favor SAB922) with a mean particle size of 450 um anda bulk density of 0.697 g/cm³ as the particulate material. It wasobserved that the absorbent grains gelled up on impregnation of theproduct with latex. The product was very brittle and was cracked.

COMPARATIVE EXAMPLE 5

The procedure described in Example 1 was repeated but using grade Ccarbon as the particulate material and using chemically bonded non-wovenmaterial as the base material (as set out in Table 5). Most of thecarbon was concentrated on one surface of the product.

DISCUSSION OF EXAMPLES AND COMPARATIVE EXAMPLES

The results for Examples 1 to 11, 12 to 17, 18 to 32 and 33 to 38 areshown in Tables 1, 2, 3 and 4 respectively (in which "-" indicates thatno data was available and ">" indicates a reading which was very high)and in FIGS. 7,8,9 and 10 respectively. This data supports the inventivestep of the present application in achieving high loadings ofparticulate material whilst using a base material of low density. IfFIGS. 7, 8, 9 and 10 were extrapolated along the X-axes to show resultsfor the loading of higher density base materials with particulatematerial, the curves would gradually level off and would then adopt apositive gradient. Conventionally, high density base materials have beenused when high powder loadings have been required since (prior to thepresent invention) the skilled man was unaware of the results for basematerials of low density and would not have regarded research into theloading of such materials with particulate material as being worthtrying. This is because the skilled man would have expected that, as thedensity of base material is reduced from the range which prior to thepresent invention was used when high powder loading was required, thepercentage particulate material loading would also fall.

Examples 39 to 41 illustrate various industrial applications of productsof the present invention. These products can be made more cheaply andmore efficiently than by using the methods of the prior art since it hasbeen found that, for a given loading of particulate material, thepresent invention requires less base material and less binder.

The Comparative Examples illustrate the advantages of the presentinvention over prior art methods and products. Comparative Examples 1aand 1b illustrate the non-suitability of the base materials described inGB-A- 1283721 for the present invention. In both of these ComparativeExample only low percentage powder loadings were achieved and theactivity of those few carbon particles which had been loaded with thebase materials was low owing to migration of binder to the surface.Furthermore, the uniformity of distribution of carbon through the basematerial was poor, with most of the carbon being concentrated on theupper surface of the material.

In Comparative Example 2, a fairly high percentage loading of carbon wasachieved using a paper base material as described in EP-A- 0272798 but,as in Comparative Examples 1a and 1b, the uniformity of distribution ofcarbon through the base material was poor with most of the carbon beingconcentrated at or around the upper surface of the material. The"blacking" of the paper sheet is effected in its low air permeability.Indeed the base paper was so tight that merely impregnating it withbinder would render it impervious (irrespective of any incorporation ofparticulate material).

In Comparative Example 3, a glass fibre mat as described in GB-A-1421346was used as the base material. This method was expensive and did notachieve a high percentage loading with particulate material. Indeed,much cheaper base material can be used to achieve this percentageloading and so this method would not be of practical application forfabric or foam base material.

Comparative Example 4 illustrates the non-applicability of the methoddescribed in GB-A- 2013102 to the present invention since the endproduct was cracked and, apart from the cracks, had hardly any air flow.

Comparative Example 5 illustrates the importance of using particulatematerial in the process of the present invention which has a mean poresize which is greater than the mean particle size of the particles whichare to be loaded on or on and in the base material. In this ComparativeExample a mean particle size of 41 um was used which was considerablylarger than the mean pore size, resulting in low retention of powder andpoor air permeability.

This situation is illustrated by FIG. 11, in which a particle 304 ofparticle size w is shown in pore 302 which has pore size x. The pore isin base material 306 and the air pressure of air adjacent to lowersurface 312 is less than of air adjacent to upper surface 310.

Particle 304 effectively blocks pore 302 leading to a lower air flowthrough the pore. Without being bound by theory, this situation isbelieved to have occurred in many of the pores of the material used inComparative Example 5, leading to a material in which most of theparticles are at or around one surface of the material and the air flowis low.

This contrasts with the process of the present invention where it isrequired that the mean pore size of the sheet material be greater thanthe mean particle size of the particulate material. The presentinvention allows concurrent control of the distribution of particles inor on and in the sheet material both over the lateral dimension of thesheet material and across its thickness.

In one embodiment of the process of the present invention, water can beused to fill or partially fill the pores adjacent to one surface of thesheet material before loading the material with particles. This allowsparticles to be located predominantly near to one surface of thematerial but does not cause substantial blockage of the pores andachieves a substantially even distribution of particles laterally acrossthe material.

    TABLE 1      BASE MATERIAL PRODUCT Ex- Description Basis Cal- Den-  Measured  Basis  A     pparent Latex Powder Powder    ample of base Weight iper sity Air Flow     at mm Pore size Weight g/m.sup.2 Caliper Density Content Content Content     Air Flow Measured at Pore Size No. material g/m.sup.2 mm. g/cm.sup.3     L/min/10 cm.sup.2 water gauge mean (μm) Bone Dry mm. g/cm.sup.3     g/m.sup.2 g/m.sup.2 % w/w L/min/10 cm.sup.2 mm. water mean (μm)       1 Chemically 95 9.8 0.01  >100   1 764 135 4.4 0.031 4.0  34 26 65  1     732  Bonded  Polyester 2 Bulked Chem- 58 3.0 0.19  >100   1 931  87 1.48     0.059 1.5  28 33 >100    1 648  ically Bonded  Polyester 3 Needled 65     3.0 0.0215 88 5 197 394 2.14 0.184 6.0 307 79 47 20 139  Polyester 4     Needled and 106  4.7 0.0225 70 2 423 552 3.6 0.153 10 430 79 70 20 313     Chemically  Bonded  Polyester 5 Mechanically 65 2.85 0.0228 105  5 229     375 1.55 0.24  10 308 84 68 20 149  Entangled  Polyester 6 Polyurethane     65 2.65 0.0245 76 50  338 407 2.6 0.16  10 337 85 23 50 242  Foam 7     Mechanically 90 3.0 0.03  36 1 402 441 2.39 0.185 16 323 76 100  50 229     Entangled  Polyester 8 Bulked Chem- 73 2.25 0.033 85 1 561 380 1.79     0.212 10 297 80 110  100  241  ically Bonded  Polyester 9 Chemically 32     0.46 0.07  25 1 197 313 0.85 0.368 8 270 89 82 100   67  Bonded     Polyester 10  Chemically 16 0.16 0.10  90 2 242 103 0.54 0.191 3  84 84     49  5 113  Bonded Viscose 11  Chemically 85 0.6 0.142 83 5 188 429 1.58     0.272 10 324 77 73 100      51  Bonded  Polyester

                                      TABLE 2                                     __________________________________________________________________________    BASE MATERIAL                              PRODUCT                            Ex- Description                                                                          Basis                                                                             Cal-                                                                             Den-        Measured     Basis                              ample                                                                             of base                                                                              Weight                                                                            iper                                                                             sity                                                                              Air Flow                                                                              at mm. Pore size                                                                           Weight g/m.sup.2                                                                     Caliper                     No. material                                                                             g/m.sup.2                                                                         mm.                                                                              g/cm.sup.3                                                                        L/min/10 cm.sup.2                                                                     water gauge                                                                          mean (μm)                                                                        Bone Dry                                                                             mm                          __________________________________________________________________________    12  Chemically                                                                           95  9.8                                                                              0.01                                                                              >100    1      764   201    5.0                             Bonded                                                                        Polyester                                                                 13  Needled                                                                              65  3.0                                                                              0.0215                                                                            88      5      197   236    1.39                            Polyester                                                                 14  Polyurethane                                                                         65  2.65                                                                             0.0245                                                                            76      50     338   226    2.56                            Foam                                                                      15  Chemically                                                                           16  0.16                                                                             0.10                                                                              90      2      242    80    0.42                            Bonded                                                                        Viscose                                                                   16  Chemically                                                                           85  0.6                                                                              0.142                                                                             83      5      189   192    0.99                            Bonded                                                                        Polyester                                                                 17  Thermally                                                                            135 0.8                                                                              0.18                                                                              51      20      74   247    1.02                            Bonded                                                                        Fabric from                                                                   heterofil                                                                     fibre                                                                     __________________________________________________________________________                 PRODUCT                                                                   Ex- Apparent                                                                           Latex                                                                              Powder                                                                             Powder       Measured at                                   ample                                                                             Density                                                                            Content                                                                            Content                                                                            Content                                                                            Air Flow                                                                              mm water                                                                             Pore Size                              No. g/cm.sup.3                                                                         g/m.sup.2                                                                          g/m.sup.2                                                                          % w/w                                                                              L/min/10 cm.sup.2                                                                     gauge  mean (μm)                  __________________________________________________________________________             12  0.0428                                                                             13   107  53   61       1     496                                    13  0.179                                                                              13   174  74   112     100    154                                    14  0.095                                                                              18   160  71   30      100    229                                    15  0.20  4   64   80   87       50    217                                    16  0.20  7   97   51   34      100    107                                    17  0.251                                                                               9   94   38   22      100     58                           __________________________________________________________________________

    TABLE 3      BASE MATERIAL PRODUCT  Description Basis  Apparent Air Flow Measured at      Basis Weight  Apparent  Powder Power   Example of base Weight Caliper     Density L/min/ mm water Pore Size g/m.sup.2 Caliper Density Latex     Content Content Content Air Flow Measured at No. material g/m.sup.2 mm     g/cm.sup.3 10 cm.sup.2 gauge mean (μm) Bone Dry mm g/cm.sup.3     g/m.sup.2 g/m.sup.2 % w/w L/min/10 cm.sup.2 mm water gauge       18 Polyester 60 12.0 0.005  >100   1 764  67 1.93 0.035  3  4  6 96  1      High Bulk  Fleece 19 Polyester 100  16.0 0.0063 >100   2 496 165 4.14     0.040 17 48 20 53  1  High Bulk  Fleece 20 Chemically 95 9.8 0.01  >100      1 764 154 4.11 0.037 22 37 28 75  2  Bonded  Polyester 21 High Bulk 208      16.2 0.013  74 5 647 345 7.78 0.044 38 99 32 100   10  Non-woven 22     Bulked 58 3.0 0.019  >100   1 931 123 1.27 0.097 20 45 44 71  1     Chemically  Bonded  Polyester 23 Needled 65 3.0 0.0215 88 5 197 372 1.52     0.245 52 255  80 10 100  Polyester 24 Mechanically 65 2.85 0.0228 105  5     229 298 1.23 0.343 44 189       74 70 100  Entangled  Polyester 25 Polyurethane 65 2.65 0.0245 76 50     338 263 2.58 0.102 42 156 71 14 100  Foam 26 Mechanically 90 3.0 0.030     36 1 402 279 2.07 0.135 36 153 63 88  10  Entangled  Polyester 27 Bulked     72 2.25 0.033  85 1 561 206 1.27 0.162 30 103 59 71  10  Chemically     Bonded  Polyester 28 Needled and 150  3.50 0.043  76 5 205 653 2.71     0.241 91 412 73  7 100  Chemically  Bonded  Polyester 29 Chemically 32     0.46 0.07  25 1 197 220  0.845 0.260 27 161 83 12 100  Bonded  Polyester     30 Chemically 16 0.16 0.10  90 2 242 165 0.80 0.204 24 123 87 12 100     Bonded  Viscose 31 Chemically 85 0.60 0.142  83 5 188 277 1.38 0.201 34     158 65 75 100  Bonded  Polyester 32 Thermally 135  0.80 0.18  51 20   74     304 1.26 0.241 33 136 50 18 100  Bonded  Fabric  from  Heterofil     Fibre

                                      TABLE 4                                     __________________________________________________________________________             2                                                                    BASE MATERIAL                                                                               Basis                   Measured                                Num-                                                                              Description of                                                                          Weight                                                                            Caliper                                                                           Apparent                                                                              Air Flow                                                                              at mm. Pore Size                        ber base material                                                                           g/m.sup.2                                                                         mm. Density g/cm.sup.3                                                                    L/min/10 cm.sup.2                                                                     water gauge                                                                          mean (μm)                     __________________________________________________________________________    33  Chemically Bonded                                                                       95  9.8 0.01    36      1      764                                  Polyester                                                                 34  Needled Polyester                                                                       65  3.0 0.0215  88      5      197                              35  Polyester 65   2.65                                                                             0.0245  76      50     338                                  Polyurethane Foam                                                         36  Chemically Bonded                                                                       16   0.160                                                                            0.10    90      2      242                                  Viscose                                                                   37  Chemically Bonded                                                                       85  0.6 0.142   83      5      188                                  Polyester                                                                 38  Thermally Bonded                                                                        135 0.8 0.18    51      20      74                                  Fabric from                                                                   Heterofil Fibre                                                           __________________________________________________________________________    GRADE CA1 PRODUCT                                                                             Apparent                                                                           Latex                                                                              Powder                                                                             Powder       Measured                          Num-                                                                              Basis Weight                                                                          Caliper                                                                           Density                                                                            Content                                                                            Content                                                                            Content                                                                            Air Flow                                                                              at mm.                            ber g/m.sup.2  Bone Dry                                                                   mm. g/cm.sup.3                                                                         g/m.sup.2                                                                          g/m.sup.2                                                                          % w/w                                                                              L/min/10 cm.sup.2                                                                     water gauge                       __________________________________________________________________________    33  127     3.715                                                                             0.034                                                                              18    14  13   60       1                                34  609     2.770                                                                             0.22 151  393  85   10      100                               35  306     3.015                                                                             0.101                                                                              76   165  72   14      200                               36  210     0.975                                                                             0.215                                                                              40   154  91   22      200                               37  297     1.74                                                                              0.171                                                                              54   158  65   17      100                               38  334     1.295                                                                             0.258                                                                              56   143  51    7      200                               __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________           BASE MATERIAL                                                          Comparative                              Measured                             Example       Basis Weight                                                                         Caliper                                                                           Apparent                                                                              Air Flow                                                                              at mm. Pore Size                     Number Material                                                                             g/m.sup.2                                                                            mm. Density g/cm.sup.3                                                                    L/min/10 cm.sup.2                                                                     water gauge                                                                          mean (μm)                  __________________________________________________________________________    1a     Cotton 85     0.14                                                                               0.208  --      --     --                                   Scrim                                                                  1b     Polypropylene                                                                        16     0.11                                                                               0.145  >       >      >                                    net                                                                    2      Paper  64      0.145                                                                            0.44     0      200    --                            3      Glass  308    0.85                                                                              0.36    90       5     --                                   Fibre Mat                                                              4      Carbon 144    0.48                                                                              0.30    33      100    256                                  Impregnated                                                                   Foam                                                                   5      Chemically                                                                           143    0.60                                                                              0.24     7      100     14                                  Bonded                                                                        Nonwoven                                                               __________________________________________________________________________           PRODUCT                                                                Comparative        Apparent                                                                           Latex                                                                              Powder                                                                             Powder       Measured                       Example                                                                              Basis Weight                                                                          Caliper                                                                           Density                                                                            Content                                                                            Content                                                                            Content                                                                            Air Flow                                                                              at mm.                         Number g/m.sup.2  Bone Dry                                                                   mm. g/cm.sup.3                                                                         g/m.sup.2                                                                          g/m.sup.2                                                                          % w/w                                                                              L/min/10 cm.sup.2                                                                     water gauge                    __________________________________________________________________________    1a     110      0.435                                                                            0.253                                                                              16    9   10   79       5                             1b 214 1.35    0.159                                                                             17   80   41   76    20                                    2      163     0.67                                                                              0.243                                                                              12   87   58    0      200                            3      387     1.18                                                                              0.328                                                                              14   65   17   85       5                             4      730     3.35                                                                              0.218                                                                              40   546  79   12      200                            5      215     0.88                                                                              0.246                                                                              42   30   17    7      200                            __________________________________________________________________________

I claim:
 1. In a method of producing a particulate-solid-bearingair-permeable sheet, said method including the step of entraining aparticulate solid in a gaseous carrier in the substantial absence offibrous material, said solid including particles having a mean particlesize, the improvement in said method comprising the steps of:providing apreformed air-permeable sheet, said sheet having one face and a secondface and being selected from non-woven fibrous materials and open cellfoam materials, which materials have a density at or below 0.25 g/cm³,said sheet having pores having a mean pore size; disposing said one faceof said sheet in the path of a stream of said gaseous carrier andentrained particulate solid, whilst maintaining a pressure drop acrossthe thickness of the preformed air-permeable sheet from said one face tosaid second face of said air-permeable sheet, the mean pore size of saidpores of the preformed air-permeable sheet being greater than the meanparticle size of the particulate solid, thereby entrapping at least partof said entrained particulate solid on or on and in said air-permeablesheet; and fixing the entrapped particular solid on or on and in theair-permeable sheet with a binder.
 2. In a method according to claim 1,the improvement further comprising:said preformed air-permeable sheethaving a density of from 0.01 to 0.18 g/cm³.
 3. In a method according toclaim 1, the improvement further comprising:said maintaining step beingperformed by maintaining a lower gaseous pressure at said second facethan that extant at said one face of said air-permeable sheet.
 4. In amethod according to claim 3, the improvement further comprising:saidmaintaining step being performed by applying suction to said second faceof said air-permeable sheet.
 5. In a method according to claim 1, theimprovement further comprising: the gaseous carrier being air.
 6. In amethod according to claim 1, the improvement furthercomprising:supplying said stream of the gaseous carrier and theparticulate solid in substantially free flowing form to a mixing zone,said mixing zone having an effective width and length, the substantiallyfree flowing particulate solid material being entrained in the mixingzone in the stream of the gaseous carrier; directing a flow of themixture of gaseous carrier and entrained particulate solid from saidmixing zone into the inlet of, through and out of the outlet of a supplyzone, said supply zone having an effective width and length; establisheda suction zone of variable effective length and width adjacent to and inline with the outlet of said supply zone, the effective width and lengthof said suction zone being greater than the effective width and lengthof said supply zone; reducing the pressure in said suction zone to belowthat at the outlet of said supply zone; and continuously feeding theair-permeable sheet between said supply zone and said suction zone. 7.In a method according to claim 6, the improvement furthercomprising:said gaseous carrier being air, establishing a recirculatoryflow of said air, wherein said recirculatory flow includes a stream ofsaid air at super atmospheric pressure flowing through the mixing zoneand then into the supply zone; maintaining said supply zone atatmospheric pressure; said reducing step causing said air from saidsupply zone to flow into the suction zone; maintaining said suction zoneat subatmospheric pressure; compressing said air from said suction zoneto superatmospheric pressure; and feeding said compressed air to saidmixing zone.
 8. In a method according to claim 1, the improvementfurther comprising:providing the particulate solid in the form of activecarbon.
 9. In a method according to claim 1, the improvement furthercomprising:the air-permeable sheet being a fibrous non-woven material.10. In a method according to claim 1, the air-permeable sheet being anopen-cell foam.
 11. In a method according to claim 1, the improvementfurther comprising:said pressure drop causing the gaseous carrier toflow through the air-permeable sheet from said one face to said secondface; and recirculating particulate solid entrained in said gaseouscarrier leaving said second face of said air-permeable sheet, saidrecirculating being by returning both a portion of said gaseous carrierleaving said second face and particulate solid leaving said second faceas an input for entraining step before said stream of gaseous carrierand entrained particulate solid reach said one face of saidair-permeable sheet.
 12. In a method according to claim 1, theimprovement further comprising:the binder being a natural or syntheticaqueous latex.
 13. In a method according to claim 1, the improvementfurther comprising:the binder being a latex and the air-permeable sheethaving the particulate solid retained thereon or thereon and thereinbeing impregnated with the latex; and drying the impregnated sheetmaterial.
 14. In a method according to claim 1, the improvement furthercomprising:said binder being a solid particulate water-soluble binder;said entraining step entraining both the particulate solid and saidsolid particulate water-soluble binder in the gaseous carrier; andimpregnating the air-permeable sheet with an amount of water sufficientto dissolve the binder and form of a binder solution in situ in theair-permeable sheet.
 15. In a method according to claim 1, theimprovement further comprising:said providing step providing saidair-permeable sheet selected from said non-woven fibrous materials andopen cell foam materials which are uncoated prior to said disposingstep.